The present invention relates to a color television camera capable of being used in an atmosphere of radiation.
In a nuclear power plant, a space-ship, etc., there is provided a monitor television camera for supervising and inspecting installations. In such a location, the dosage of radiation is large in comparison with an ordinary location and the performance of a television camera can not be secured due to the influence of radiation if an ordinary television camera is employed.
For example, if a color television camera in which an image pickup tube, an amplifier, and a control circuit are integrated is subject to gamma-ray exposure, deterioration begins to occur in color reproducibility and in luminance signal level at the dosage of 10.sup.3 Rad, and the image pickup function vanishes at 10.sup.4 Rad, so that it can not be used in the circumstances of high dosage not smaller than 10.sup.3 -10.sup.4 ("high dosage" is defined as a dosage not smaller than 10.sup.3 -10.sup.4).
The causes of such deterioration in performance in a television camera due to radiation are as follows:
(I) Browning in lens and glass used in an image pickup tube due to radiation dosage:
In a color television camera, a color information of an object to be taken up is decomposed into the three primary colors (red component (R), green component (G), blue component (B)) so as to form a video signal. If browning occurs in a lens or a face plate of the image pickup tube, the transmittivity of the respective hue components decreases. The decrease in transmittivity of the blue component is especially remarkable.
(II) Deterioration in an SN ratio in a chrominance component processing circuit, especially in amplifiers, due to radiation exposure:
Noises due to radiation exposure are generated in semiconductor devices per se in a chrominance component processing circuit. Especially in an amplifier, the amplification of an amplifier element decreases so that the level of the signal component falls down and the SN ratio is deteriorated.
Generally, a color television camera is adjusted in advance such that the colors of picked-up image of the object can be reproduced into colors to be observed with the naked eye as faithfully by as possible. If the camera is exposed to radiation, the reproducibility of the camera may be deteriorated by the causes as described above.
Further, in a color television camera in which the frequency interleave system is employed as the color image pickup method, deterioration may occur in color reproducibility also by the following cause:
(III) Change in scanning speed (scanning frequency) of saw tooth wave of a horizontal deflection circuit:
In a color television camera of the frequency interleave type, color information is extracted through stripe filters on a face plate glass and separated into respective color information through filters corresponding to stripe frequencies. If the scanning frequency changes the time-base frequency of the stripe color filter changes, though the spatial frequency of the same does not change. Consequently, the chrominance component passing through a band pass filter for separating the chrominance component from a video signal decreases. Similarly to the cause (II) as described above, this change in scanning speed is caused by the influence of radiation onto semiconductor devices.
Conventionally, pure silica glass or radiation resistant glass doped with Ce (cerium) has been employed as lenses or as various kinds of glass used for an image pickup tube and bipolar elements having radiation resistance superior to MOS-FETs, or the like, have been used as circuit elements, thereby providing radiation resistance of the total color television camera.
In spite of such a countermeasure, however, the browning in glass might progress to deteriorate the color reproducibility if the color television camera was continuously used in an atmosphere of radiation. Further, such a restriction that bipolar elements have to be used as circuit elements caused problems that there occurred a difficulty in reduction in size of the circuit and that current consumption increased. Furthermore, in fact, it has been difficult to sufficiently prevent the deterioration from occurring even if such bipolar elements were used. Thus, there was a limit in the countermeasure to maintain the initial performance (that is, color reproducibility) for a long time only by constituting the elements composing a camera by using materials of relatively strong radiation resistance.